<html>
<body>
  <div class="outside-shadow-dom">
    <div class="outside-shadow-dom">
      <div class="shadow-host"></div>
    </div>
  </div>

  <script>
    document
    .querySelector('.shadow-host')
    .attachShadow({ mode: 'open' })
    .innerHTML = '<div class="in-shadow-dom">In the Shadow DOM</div>'
  </script>
</body>
</html>
